1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell stack formed by stacking a plurality of unit cells in a horizontal direction. Each of the unit cells includes an electrolyte electrode assembly and separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. Six fluid passages extend through the unit cells in the stacking direction. Three of the six fluid passages are provided on the left of the unit cells, and the other three of the six fluid passages are provided on the right of the unit cells.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly which includes an anode and a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. Each of the anode and the cathode is made of electrode catalyst layer of noble metal formed on a base material chiefly containing carbon. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a fuel cell for generating electricity.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen or air (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Generally, a predetermined number of, e.g., several tens to several hundreds of fuel cells are stacked together to form a fuel cell stack for achieving the desired level of electricity in power generation. The fuel cell is considerably long in the stacking direction. Therefore, it is not possible to supply the fuel gas equally to each of the fuel cells. In order to address the problem, a fuel cell system including a plurality of juxtaposed fuel cell stacks has been proposed. For example, International Patent Publication No. 96/20509 titled “INTEGRATED EXTERNAL MANIFOLD ASSEMBLY FOR AN ELECTROCHEMICAL FUEL CELL STACK ARRAY” is known. In the conventional technique, as shown in FIG. 9, an electrochemical fuel cell stack array 1 includes four fuel cell stacks 2a through 2d. Each of the fuel cell stacks 2a through 2d is formed by stacking a plurality of unit cells 3 in a stacking direction indicated by an arrow X.
The electrochemical fuel cell stack array 1 is connected to external manifold assemblies 4. The external manifold assemblies 4 include a supply manifold comprising main pipes 5a, 5b, 5c, and a discharge manifold comprising main pipes 6a, 6b, 6c. 
The fuel gas, the oxygen-containing gas, and the coolant flow through the main pipes 5a through 5c. For example, a plurality of manifold bifurcated pipes 7 for supplying the reactant gases to each of the fuel cell stacks 2a through 2d are connected to the manifold main pipe 5a. 
Likewise, the fuel gas, the oxygen-containing gas, and the coolant flow through the main pipes 6a through 6c. For example, a plurality of manifold bifurcated pipes 8 for discharging the reactant gases from each of the fuel cell stacks 2a through 2d are connected to the manifold main pipe 6a. 
However, in the conventional technique, since the external manifold assemblies 4 are provided on opposite ends of the electrochemical fuel cell stack array 1 in the direction indicated by the arrow X, the overall size of the electrochemical fuel cell stack array 1 is considerably large. The piping system is complicated and large. Thus, the operation of connecting the pipes is laborious, and the electrochemical fuel cell stack array 1 cannot be installed in a small space. In particular, the electrochemical fuel cell stack array 1 is not suitable for use in a vehicle.
Further, the manifold bifurcated pipes 7 branched from the main pipe 5a and the manifold bifurcated pipes 8 branched from the main pipe 6a have different lengths corresponding to the supply ports and discharge ports of the respective fuel cell stacks 2a through 2d. Therefore, it is not possible to provide the reactant gases to each of the fuel cell stacks 2a through 2c uniformly.